Liner-free conductive structures

ABSTRACT

The present disclosure describes a method for forming liner-free or barrier-free conductive structures. The method includes forming a liner-free conductive structure on a cobalt conductive structure disposed on a substrate, depositing a cobalt layer on the liner-free conductive structure and exposing the liner-free conductive structure to a heat treatment. The method further includes removing the cobalt layer from the liner-free conductive structure.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a divisional of U.S. patent application Ser.No. 16/887,577, filed on May 29, 2020 and titled “Liner-free ConductiveStructure,” which is incorporated by reference herein in its entirety.

BACKGROUND

In an integrated circuit, conductive structures (e.g., metal contacts,vias, and lines) are electrically coupled to transistor regions, such asthe gate electrode and the source/drain terminals, and are configured topropagate electrical signals from and to the transistors. The conductivestructures, depending on the complexity of the integrated circuit, canform one or more layers of metal wiring.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures.

FIG. 1 is a cross-sectional view of a structure with barrier-free orliner-free conductive structures thereon, in accordance with someembodiments.

FIGS. 2A and 2B are flowcharts of a method describing the formation ofbarrier-free or liner-free conductive structures, in accordance withsome embodiments.

FIGS. 3-7 are cross-sectional views of various fabrication operationsduring the formation of barrier-free or liner-free conductivestructures, in accordance with some embodiments.

FIG. 8A is a magnified cross-sectional view of a via opening over acobalt conductive structure, in accordance with some embodiments.

FIG. 8B, is a magnified top view of a via opening over a cobaltconductive structure, in accordance with some embodiments.

FIGS. 9 and 10 are magnified cross-sectional views of variousfabrication operations during the formation of a barrier-free orliner-free conductive structure, in accordance with some embodiments.

FIG. 11 is magnified cross-sectional view of a cobalt layer formed on abarrier-free or liner-free conductive structure, in accordance with someembodiments.

FIG. 12 is magnified cross-sectional view of a cobalt layer formed on abarrier-free or liner-free conductive structure during a heat treatment,in accordance with some embodiments.

FIG. 13 is magnified cross-sectional view of a non-coalesced cobaltlayer formed on a barrier-free or liner-free conductive structure, inaccordance with some embodiments.

FIG. 14 is a cross-sectional view of metallization layers overbarrier-free or liner-free conductive structures, in accordance withsome embodiments.

FIGS. 15A-C are magnified cross-sectional views of various fabricationoperations during the formation of a barrier-free or liner-freeconductive structure, in accordance with some embodiments.

FIGS. 16A-C are magnified cross-sectional views of various fabricationoperations during the formation of a barrier-free or liner-freeconductive structure, in accordance with some embodiments.

FIGS. 17A-C are magnified cross-sectional views of various fabricationoperations during the formation of a barrier-free or liner-freeconductive structure, in accordance with some embodiments.

FIGS. 18A-C are magnified cross-sectional views of various fabricationoperations during the formation of a barrier-free or liner-freeconductive structure, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature on a second feature in the description that followsmay include embodiments in which the first and second features areformed in direct contact, and may also include embodiments in whichadditional features are disposed between the first and second features,such that the first and second features are not in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition does not in itselfdictate a relationship between the various embodiments and/orconfigurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The term “nominal” as used herein refers to a desired, or target, valueof a characteristic or parameter for a component or a process operation,set during the design phase of a product or a process, together with arange of values above and/or below the desired value. The range ofvalues can be due to slight variations in manufacturing processes and/ortolerances.

In some embodiments, the terms “about” and “substantially” can indicatea value of a given quantity that varies within 5% of the value (e.g.,±1%, ±2%, ±3%, ±4%, ±5% of the value). These values are merely examplesand are not intended to be limiting. It is to be understood that theterms “about” and “substantially” can refer to a percentage of thevalues as interpreted by those skilled in relevant art(s) in light ofthe teachings herein.

Active and passive devices in an integrated circuit (IC) areinterconnected at a local level (e.g., within the same area of the IC)and at a global level (e.g., between different areas of the IC) througha number of conductive structures, such as metal contacts, metal vias,and metal lines. These conductive structures—which can include differentconductive materials (e.g., a different metal fill)—are formed invertically stacked metallization layers. Design considerations are takeninto account when metallization layers with different conductivematerials are stacked on top of each other to avoid performancedegradation due to any unwanted interaction between the conductivematerials. This is especially the case for conductive structures withoutbarrier or liner layers.

Conductive structures without barrier or liner layers (also referred toherein as “liner-free or barrier-free conductive structures” can havelower electrical resistance compared to conductive structures withbarrier or liner layers. This is because liner or barrier layers whichcan be more resistive than a metal fill layer consume area within theconductive structure. Therefore, by eliminating the liner or barrierlayers in the conductive structures, the lower resistance metal fill canoccupy the entire volume of the conductive structure and reduce theoverall contact resistance of the conductive structure.

In the absence of liner or barrier layers, liner-free or barrier-freeconductive structures formed in contact with conductive structuresfilled with a different metal may be unable to prevent out-diffusion ofthe underlying metal under certain conditions. For example, rutheniumfilled liner-free or barrier-free conductive structures overlying acobalt conductive structure are unable to prevent cobalt out-diffusionthrough the ruthenium metal grain boundaries when both structures areannealed. Cobalt out-diffusion results in voids within cobalt conductivestructure. The aforementioned behavior poses limitations to theimplementation of ruthenium filled liner-free or barrier-free conductivestructures and makes the ruthenium filled liner-free or barrier-freeconductive structures challenging to integrate with cobalt conductivestructures.

To address the aforementioned shortcomings, this disclosure describesmethods directed to the suppression of cobalt out-diffusion fromunderlying cobalt conductive structures to overlying ruthenium-filledliner-free or barrier-free conductive structures. In some embodiments, acobalt layer formed on the ruthenium conductive structures acts as a“reservoir” for cobalt atoms that mitigates out-diffusion of cobaltatoms from underlying cobalt conductive structures. In some embodiments,the cobalt layer is a sacrificial layer subsequently removed prior tothe formation of additional wiring levels. In some embodiments, aportion of the cobalt layer is incorporated in the ruthenium-filledliner-free or barrier-free conductive structures. In some embodiments,cobalt atoms diffuse from both the overlying cobalt layer and theunderlying cobalt conductive structures into the ruthenium-filledliner-free or barrier-free conductive structures to fill voids or grainboundaries within the ruthenium metal fill. In some embodiments, theruthenium-filled liner-free or barrier-free conductive structures arerecessed with an etch-back process prior to the formation of the cobaltlayer. In some embodiments, the liner-free or barrier-free conductivestructures are partially filled with ruthenium prior to the formation ofthe cobalt layer.

According to some embodiments FIG. 1 is a cross-sectional view of astructure with liner-free or barrier-free conductive structure 100 (alsoreferred to herein as “liner-free conductive structure 100”) formed on acobalt conductive structure 105. In some embodiments, the liner-freeconductive structure 100 is filled with ruthenium metal 110. As shown inFIG. 1, cobalt conductive structure 105 is formed on a mergedsource/drain epitaxial structure 115 grown on fin structures 120, whichare in turn disposed on a substrate 125. In some embodiments, finstructures 120 and the bottom portion of source/drain epitaxial layer115 are surrounded by a first dielectric layer 130, while the upperportion of source/drain epitaxial layer 115 and cobalt conductivestructure 105 are surrounded by a second dielectric layer 135. In someembodiments, first dielectric layer 130 forms an isolation structure,such a shallow trench isolation (STI).

The top and mid-sections of liner-free conductive structure 100 (e.g.,above cobalt conductive structure 105) are surrounded by an etch stoplayer 140 and an interlayer dielectric (ILD) 145. In contrast, bottomsections of liner-free conductive structure 100 (e.g., below the topsurface of cobalt conductive structure 105) are embedded in cobaltconductive structure 105. The bottom sections of liner-free conductivestructure 100 (e.g., within cobalt conductive structure 105) aresemi-spherically shaped and each forms an “anchor point” 150 thatprevents ruthenium metal 110 from being “pulled-out” during a subsequentruthenium planarization process. Anchor point 150 also increases thesurface area between ruthenium metal 110 and cobalt conductive structure105 to reduce the contact resistance between the two structures. In someembodiments, a silicide layer 155 is interposed between cobaltconductive structure 105 and source/drain epitaxial structure 115 toreduce the electrical resistance between cobalt conductive structure 105and source/drain epitaxial structure 115.

The structures shown in FIG. 1 are exemplary and variations are withinthe spirit and the scope of this disclosure. For example, each finstructure 120 can have its own source/drain epitaxial structure insteadof a single merged source/drain epitaxial structure 115. Further,additional or fewer liner-free conductive structures, like liner-freeconductive structure 100, can be formed on cobalt conductive structure105 or other cobalt conductive structures not shown in FIG. 1.Additional or fewer fin structures 120 can also be formed on substrate125. Further, FIG. 1 shows selective portions of the structures andother portions are not shown for simplicity. For example, liner layers,barrier layers, or adhesion layers for cobalt structure 105 are notshown in FIG. 1. Further, a gate structure formed on fin structures 120adjacent to source/drain epitaxial structure 115 along the x-direction,spacer structures, doped regions, and capping layers for source/drainepitaxial structure 115 and fin structures 120 are not shown.

In some embodiments, cobalt structure 105 is a source/drain contact onwhich liner-free conductive structure 100 is formed without interveninglayers, such as barrier layers, liner layers, or adhesion layers. Insome embodiments, liner-free conductive structures, like liner-freeconductive structure 100, form a network of vertical contacts thatelectrically connect cobalt structures, like cobalt structure 105, toupper metallization levels (e.g., to copper metallization levels) notshown in FIG. 1 for simplicity. According to some embodiments,liner-free conductive structure 100 is formed with a process thatmitigates cobalt out-diffusion to the upper metallization levels throughliner-free conductive structure 100. Cobalt out-diffusion creates voidsin cobalt conductive structure 105 and can result in resistancedegradation (e.g., a resistance increase of up to about 15%). In somecases, cobalt out-diffusion, if allowed, results in electrical openswithin the cobalt conductive structure 105.

In some embodiments, FIGS. 2A and 2B are flowcharts of a fabricationmethod 200 for the formation of liner-free conductive structure 100shown in FIG. 1. Other fabrication operations may be performed betweenthe various operations of method 200 and may be omitted merely forclarity and ease of description. These various operations are within thespirit and the scope of this disclosure. Additionally, not alloperations may be required to perform the disclosure provided herein.Some of the operations may be performed simultaneously, or in adifferent order than the ones shown in FIGS. 2A and 2B. In someembodiments, one or more other operations may be performed in additionto or in place of the presently described operations.

Liner-free conductive structure 100 fabricated with method 200 is notlimited to cobalt source/drain contacts. For example, liner-freeconductive structure 100 can be formed using method 200 oncobalt-containing gate contacts or any other type of cobalt conductivestructure used in integrated circuits.

In some embodiments, FIG. 3 is an intermediate structure for method 200.in FIG. 3, fin structures 120, first dielectric layer 130, source/drainepitaxial structure 115, silicide layer 155, cobalt conductive structure105, and second dielectric layer 135 have been previously formed inoperations not shown in method 200. In some embodiments, FIG. 3 showsthe structure of FIG. 1 after the formation of cobalt conductivestructure 105 on silicide layer 155 over source/drain epitaxialstructure 115. At the fabrication stage shown in FIG. 3, the top surfaceof cobalt conductive structure 105 is substantially coplanar with thetop surface of second dielectric layer 135. This can be achieved with,for example, a planarization process after the deposition of cobaltmetal.

In referring to FIG. 2A, method 200 begins with operation 210 and theprocess of depositing an etch stop layer (e.g., like etch stop layer 140shown in FIG. 1) on an underlying conductive structure, like cobaltconductive structure 105. Etch stop layer 140 can be blanket depositedto cover cobalt conductive structure 105 and second dielectric layer 135as shown in FIG. 4. In some embodiments, etch stop layer 140 facilitatesthe formation of liner-free conductive structure 100. In someembodiments, etch stop layer 140 can include silicon nitride (Si₃N₄),silicon oxynitride (SiON), silicon carbide (SiC), silicon carbo-nitride(SiCN), or any combination thereof. Further, etch stop layer 140 can bedeposited by low pressure chemical vapor deposition (LPCVD), plasmaenhanced chemical vapor deposition (PECVD), chemical vapor deposition(CVD), atomic layer deposition (ALD), or any other suitable depositionprocess at a thickness between about 1 nm and about 3 nm.

In referring to FIGS. 2A and 5, method 200 continues with operation 220and the process of depositing ILD 145 on etch stop layer 140. In someembodiments, ILD 145 is a carbon-doped silicon oxide containing hydrogenand nitrogen. By way of example, ILD 145 can be deposited by (CVD),plasma-assisted CVD, PECVD, Or any other suitable deposition method. Insome embodiments, ILD 145 is a low-k dielectric material with adielectric constant lower than about 3.9. In some embodiments, ILD 145can be deposited at a thickness between about 50 nm and about 70 nmdepending on the desired aspect ratio of the liner-free conductivestructure.

In referring to FIG. 2A, method 200 continues with operation 230 and theprocess of forming an opening (e.g., a via opening) in ILD 145 and etchstop layer 140 to expose cobalt conductive structure 105. In someembodiments, one or more openings can be formed concurrently to exposeportions of cobalt conductive structure 105. In some embodiments, FIG. 6shows the structure of FIG. 5 after the formation of opening 600according to operation 230. In some embodiments, opening 600 is a viaopening that can be formed with photolithography and etching operations.For example, a photoresist layer can be deposited on ILD 145 andsubsequently patterned to form an etch mask. A dry etching processetches portions of ILD 145 and etch stop layer 140 not covered by thephotoresist etch mask to form opening 600 shown in FIG. 6. Since ILD 145and etch stop layer 140 include different materials, a dry chemistrywith different etch selectivity for the etched layers can be used. Insome embodiments, the dry etch includes two or more sub-operations. Forexample, a first sub-operation etches ILD 145 and terminates on etchstop layer 140, and a second sub-operation etches etch stop layer 140and terminates on cobalt conductive structure 105. Additionalsub-operations can be used to over-etch cobalt conductive structure 105and/or to remove a polymer formed during the etching processes. In someembodiments, ILD 145 and etch stop layer 140 can be etched with adifferent etching chemistry.

In some embodiments, sidewall angle θ measured between a sidewall ofopening 600 and horizontal direction y (e.g., parallel to the topsurface of cobalt conductive structure 105), is modulated via theetching process conditions and can range from about 85° to about 90°.Therefore, the top width of opening 600 can be substantially equal to orlarger than the bottom width of opening 600. In some embodiments, theaspect ratio (e.g., height/top width) of opening 600 can range betweenabout 3 and about 4. This is not limiting and more aggressive or lessaggressive aspect ratios are possible.

In referring to FIG. 2A, method 300 continues with operation 240 and theprocess of etching the exposed cobalt conductive structure 105 with awet etching process to form an anchor recess. In some embodiments, FIG.7 shows the structure of FIG. 6 after the wet etching process accordingto operation 240. In some embodiments, the etching chemistry includes anaqueous solution of butoxyethanol (C₆H₁₄O₂), hydroxylamine (H₃NO), anddiethylenetriaminepentaacetic acid (C₁₄H₂₃N₃O₁₀), in which the mainetchant is water while C₆H₁₄O₂, H₃NO, and C₁₄H₂₃N₃O₁₀ function as cobaltsurface protectants. The wet etching chemistry is selective to cobaltand isotropically etches the exposed cobalt metal in all directions(e.g., x-, y-, and z-directions). As a result, a semi-spherical anchorrecess 700 is formed on a top portion of cobalt conductive structure 105as shown in FIG. 7.

In some embodiments, the exposure of cobalt conductive structure 105 tothe wet etching chemistry is timed to control the size of semi-sphericalanchor recess 700. For example, the exposure time can range from about50 s to about 100 s or more depending on the etch rate and the desiredsize of semi-spherical anchor recess 700. FIG. 8A is a magnified view ofsemi-spherical anchor recess 700 included within dashed box 705 shown inFIG. 7. In some embodiments, semi-spherical anchor recess 700 has awidth A along the y-direction between about 21 nm and about 39 nm and aheight H between about 7 nm and about 13 nm. In some embodiments, aratio A/H is about 3. The aforementioned ranges are not limiting and alarger or a smaller semi-spherical anchor recess 700 can be formed. Insome embodiments, a larger semi-spherical anchor recess 700 can beharder to fill in a subsequent operation and a smaller semi-sphericalanchor recess 700 may not prevent metal pull-out. According to someembodiments, width A of semi-spherical anchor recess 700 is larger thanbottom width B of opening 600 (e.g., A>B), which ranges between about 13nm and about 15 nm. In some embodiments, a ratio A/B ranges betweenabout 1.7 and about 2.6, and a ratio B/H ranges between about 1 and 2.An undercut having a width along the y-direction of about (A-B)/2 isformed on each side of semi-spherical anchor recess 700 below etch stoplayer 140. In some embodiments, the undercut ranges between about 4 nmand about 12 nm.

In some embodiments, the width of semi-spherical anchor recess 700 alongthe x-direction (not shown in FIG. 8A) can be different than width Aalong the y-direction due to the smaller dimensions of cobalt structure105 along the x-direction. This is shown in FIG. 8B, which is a top-viewof semi-spherical anchor recess 700 through opening 600, where width Cof semi-spherical anchor recess 700 along the x-direction is restrictedby the physical width of cobalt conductive structure 105 and is smallerthan width A along the y-direction. In other words, the anchor recessappears to have a semi-spherical shape when viewed perpendicular towidth C (e.g., like in FIG. 8A) and a rectangular shape when viewed in across-section perpendicular to width C.

Semi-spherical anchor recess 700 can serve two purposes: (1) offer ananchor point for the metal fill to prevent pull-out of the metal fill(e.g., used to fill opening 600) during a subsequent planarizationprocess, and (2) increase the contact area between the underlying cobaltconductive structure and the metal fill to improve the overall contactresistance.

In referring to FIG. 2B, method 200 continues with operation 250 and theprocess of depositing a metal to fill opening 600. In some embodiments,the metal in operation 250 is directly deposited on cobalt structure 105without a prior deposition of a liner or a barrier layer. In someembodiments, the metal in operation 250 includes ruthenium depositedwith a thermal CVD process at a temperature below about 200° C. (e.g.,about 180° C.) using a ruthenium carbonyl precursor chemistry, such astriruthenium dodecacarbonyl (Ru₃(CO)₁₂). In some embodiments, theruthenium metal is deposited at a thickness of about 20 nm or atthickness sufficient to substantially fill opening 600, includingsemi-spherical anchor recess 700. According to some embodiments, FIG. 9is a magnified view of FIG. 8A after operation 250 and the deposition ofruthenium metal 110 in fill opening 600. In some embodiments, theas-deposited ruthenium metal 110 extends over ILD 145 outside opening600 and forms an “overburden” which is removed with a planarizationprocess in a subsequent operation.

The deposited ruthenium metal 110 has a polycrystalline microstructureconsisting of coalesced ruthenium grains with grain boundaries 900formed between abutting ruthenium grains. In some embodiments, rutheniummetal 110 may also include fill voids 910 formed at or near the grainboundary locations and/or on sidewall surfaces as shown in FIG. 9. Thelocation, the number, and the size of grain boundaries 900 and fillvoids 910 shown in FIG. 9 are not limiting. Therefore, additional grainboundaries 900 and fill voids 910 formed at different locations and withdifferent sizes/shapes are within the spirit and the scope of thisdisclosure.

In some embodiments, subsequent processes having sufficient thermalbudget (e.g., greater than about 250° C.) can result in thermalout-diffusion of cobalt atoms from cobalt conductive structure 105 toruthenium metal 110 through the ruthenium grain boundaries 900. In someembodiments, thermally diffused cobalt atoms accumulate along grainboundaries 900 and/or in fill voids 910. Thermally out-diffused cobaltatoms described above form voids within cobalt conductive structures105. For example, the voids in cobalt conductive structure 105 caused bythe cobalt out-diffusion process can have a width (e.g., along thex-direction, the y-direction, or combinations thereof) of about 55 nmand a height (e.g., in the z-direction) of about 26 nm. Aside fromcobalt voids within cobalt conductive structure 105, cobaltout-diffusion is not desirable because diffused cobalt increases theoverall contact resistance.

In some embodiments, ruthenium metal 110 is planarized so that the topsurface of ruthenium metal 110 and the top surface of ILD 145 aresubstantially coplanar as shown in FIG. 10. In some embodiments, theplanarization process is a chemical mechanical polishing (CMP) processthat removes excess ruthenium metal deposited on ILD 145 and planarizesthe top surface of the resulting structure. In some embodiments, theplanarization process reduces the height of ILD 145 to about half, orless than half, of its original height. For example, if the originalheight of ILD 145 was about 50 nm, the height of ILD 145 after theplanarization process of operation 250 can be reduced to about 30 nm orless (e.g., to about 20 nm). This height reduction can change the aspectratio of the formed liner-free conductive structure 100.

In referring to FIGS. 2B and 11, method 200 continues with operation 260and the process of depositing a cobalt layer 1100 on the metal fill(e.g., ruthenium metal 110). In some embodiments, the thickness of thedeposited cobalt layer 1100 is between about 5 nm and about 20 nm. Insome embodiments, capping layer 1100 can be deposited with aplasma-enhanced chemical vapor deposition (PECVD) process or anothersuitable deposition process at a temperature range between about 160° C.and about 260° C. using a cobalt carbonyl precursor (e.g.,cyclopentadienylcobalt dicarbonyl) and ammonia (NH₃) plasma.

In referring to FIG. 2B, method 200 continues with operation 270 and theprocess of exposing ruthenium metal 110 to a heat treatment In someembodiments, the heat treatment is performed at a temperature greaterthan about 250° C. (e.g., at about 300° C.). In some embodiments, theannealing ambient includes nitrogen (N₂), argon (Ar), helium (He),hydrogen (H₂), forming gas (e.g., a mixture of hydrogen and nitrogen),or any combinations thereof. An oxidizing ambient is not desirablebecause it can partially convert ruthenium metal 110 to ruthenium oxide,which has a higher electrical resistivity than ruthenium metal. Inreferring to FIG. 12, during the heating treatment indicated by wavylines 1200, cobalt atoms from cobalt layer 1100 diffuse into rutheniummetal 110 as indicated by arrows 1210. Cobalt diffusion can also occurfrom cobalt conductive structure 105 as indicated by arrows 1220. Asdiscussed above, grain boundaries 900 become diffusion pathways for thethermally diffused cobalt atoms. In some embodiments, diffused cobaltatoms accumulate along grain boundaries 900 and within fill voids 910 ofruthenium metal 110. According to some embodiments, the flux of diffusedcobalt atoms from cobalt conductive structure 105 is substantiallyreduced since cobalt layer 1100 functions as a secondary reservoir forcobalt atom diffusion. Consequently, saturation within ruthenium metal110 is reached sooner, and void formation within cobalt conductivestructure 105 is prevented or substantially mitigated.

In some embodiments, cobalt out-diffusion is not layer thicknessdependent. Therefore, a cobalt layer 1100 with a thickness greater thanabout 20 nm does not offer additional cobalt out-diffusion benefits; tothe contrary, a cobalt layer 1100 with a thickness greater than about 20nm unnecessarily increases the fabrication cost.

On the other hand, cobalt layers thinner than about 5 nm do not form acontinuous layer. For example, in referring to FIG. 13, cobalt grains1300 are not grown to the point where they can coalesce (e.g., merge)into a continuous layer when the cobalt layer growth is interruptedbefore it reaches a thickness of about 5 nm. In some embodiments, duringoperation 270, cobalt atoms from a non-coalesced cobalt “layer” arestill capable of out-diffusing towards ruthenium metal 110 like a fullycoalesced layer, such as cobalt layer 1100 shown in FIG. 12. Therefore,in some embodiments, cobalt layer 1100 shown in FIG. 11 is not requiredto be continuous. For example, cobalt layer 1100 can be a contiguouslayer formed by cobalt grains 1300 (e.g., as shown in FIG. 13) and havea cobalt “layer” thickness less that about 5 nm.

In some embodiments, the heat treatment of operation 270 also promotesthe growth of the ruthenium grains within ruthenium metal 110 and,consequently, reduces the number of grain boundaries 900. Grain growthis desirable because ruthenium metal with larger grains exhibits a lowerelectrical resistance compared to ruthenium metal with smaller grains.This is because a metal with large grains has fewer grainboundaries<e.g., locations for electron scattering—compared to a metalwith small grains. In some embodiments, as the grains of ruthenium metal110 grow, fill voids 910 are eliminated or undergo a size reduction, thenumber of grain boundaries 900 reduces, and cobalt out-diffusion fromcobalt layer 1100 and cobalt conductive structures 105 ceases.Therefore, out-diffusion of cobalt from cobalt layer 1100 and cobaltconductive structures 105 is restricted both because it reaches asaturation point, and because the number of grain boundaries 900 andfill voids 910 reduces during the heat treatment of operation 270.

In some embodiments, a critical parameter is the volume ratio betweenopening 600 shown in FIG. 8A and cobalt 105 (also referred to herein as“volume ratio 600/105”). For example, a volume ratio 600/105 less thanabout 0.2 may not require a cobalt layer 1100 since cobalt out-diffusionform cobalt conductive structure 105 is limited and voids within cobaltconductive structure 105 can be avoided during the heat treatmentaccording to operation 270. For a volume ratio 600/105 greater thanabout 0.2, the cobalt layer 1100 formation may be necessary to reducecobalt out-diffusion form cobalt conductive structure 105 and theformation of voids within cobalt conductive structure 105. In someembodiments, if the volume ratio 600/105 is greater than about 0.8, thevolume of ruthenium metal 110 will need to be reduced to avoid excessivecobalt out-diffusion as will be discussed below with respect to FIGS.16A-C, 17A-C, and 18A-C.

In referring to FIG. 2B, method 200 continues with operation 280 and theprocess of removing cobalt layer 1100. In some embodiments, cobalt layer1100 is removed from ILD 145 and ruthenium metal 110 with aplanarization process, such as a CMP process, or an etching processselective towards cobalt e.g., a wet etching process, a dry etchingprocess, or combinations thereof.

In referring to FIG. 2B, method 200 continues with operation 290 and theprocess of forming one or more metallization layers on ILD 145. In someembodiments, FIG. 14 shows the structure of FIG. 1 after operation 280where metallization layers 1400 and 1410 are successively formed on 145.According to some embodiments, metallization layers 1400 and 1410 areback-end-of-line (BEOL) metallization layers, which include copperconductive structures 1420 and 1430 embedded respectively in dielectriclayers 1440 and 1450. Metallization layers 1400 and 1410 further includemetal oxide etch stop layers 1460 and 1470 which facilitate theformation of copper conductive structures 1420 and 1430.

In some embodiments, copper conductive structures 1420 and 1430 includecopper fill 1480 surrounded by a liner layer 1490. In some embodiments,liner layer 1490 can include a barrier layer (e.g., tantalum nitride(TaN)) and a metal layer (e.g., tantalum or cobalt) on which copper fill1480 can be formed. According to some embodiments, dielectric layers1440 and 1450 include a stack of dielectric layers, such as a low-kdielectric and another dielectric. For example, dielectric layers 1440and 1450 can include: (i) a low-k dielectric (e.g., carbon-doped siliconoxide) and a silicon carbide with nitrogen doping; (ii) a low-kdielectric (e.g., carbon-doped silicon oxide) and a silicon carbide withoxygen doping; (iii) a low-k dielectric (e.g., carbon doped siliconoxide) with silicon nitride; or (iv) a low-k dielectric (e.g.,carbon-doped silicon oxide) with silicon oxide.

In some embodiments, metal oxide etch stop layers 1460 and 1470 have athickness of about 3 nm and can include, for example aluminum oxide(Al₂O₃). In some embodiments, metal oxide etch stop layers 1460 and1470, besides facilitating the formation of copper conductive structures1420 and 1430, are able to suppress cobalt out-diffusion from the topsurface of liner-free conductive structure 100 during subsequent thermalprocesses (e.g., during subsequent material depositions, thermaltreatments, wet cleans, etching operations, etc.) performed attemperatures greater than about 250° C.

In some embodiments, instead of polishing down the as-depositedruthenium metal 110 as shown in FIGS. 9 and 10, ruthenium metal 110 canbe selectively etched-back as shown in FIG. 15A. For example, anetch-back process may be preferred over a polishing process if thevolume ratio 600/105 is greater than about 0.8 as discussed above.Because of the etch-back process, ruthenium metal 110 is recessed by arecess height R1 with respect to the surrounding ILD 145. In someembodiments, recess height R1 is controlled via the etch-back processconditions, such as the etching time. In some embodiments, the recessheight H1 is adjusted so that the recessed ruthenium metal 110 shown inFIG. 15A has a height H1 that is substantially equal to height H2 ofpolished or planarized ruthenium metal 110 shown in FIG. 10 (e.g.,H1≈H2).

In some embodiments, the etch-back process includes a wet etchingchemistry, such as hypochlorous acid (HClO). In some embodiments, theetch-back process, due to its isotropic nature, forms a concave topsurface on ruthenium top metal 110 as shown in FIG. 15A.

Subsequently, cobalt layer 1100 is blanket deposited on the recessed(e.g., etched) ruthenium metal 110 as shown in FIG. 15B. The structureis subjected to a heat treatment as described in operation 270 of method200 shown in FIG. 2B so that cobalt from cobalt layer 1100 can diffuseinto the recessed ruthenium metal 110. A CMP process planarizes (e.g.,polishes down) cobalt layer 1100 and a portion of ILD 145 to formliner-free or barrier-free conductive structure 100 with height H1 asshown in FIG. 15C. Because of the aforementioned CMP process, the topsurface conductive structure 100 is coplanar with the top surface of ILD145 as shown in FIG. 15C.

In some embodiments, the etch-back process recesses ruthenium metal to aheight H3 as shown in FIG. 16A shorter than height H2 shown in FIG. 10(e.g., H3<H2). Recess height R2 is taller than recess height R1 (e.g.,R2>R1). Subsequently, cobalt layer 1100 is blanket deposited on therecessed ruthenium metal 110 as shown in FIG. 16B. The structure issubjected to a heat treatment as described in operation 270 of method200 shown in FIG. 2B so that cobalt from cobalt layer 1100 can diffuseinto the recessed ruthenium metal 110. A CMP process planarizes (e.g.,polishes down) cobalt layer 1100 and a portion of ILD 145 as shown inFIG. 16C to form conductive structure 100 with a height H4—substantiallyequal to height H2 shown in FIG. 10. A difference between FIGS. 15C and16C is that in the case of FIG. 16C a portion of cobalt layer 1100 isintegrated in the resulting conductive structure.

Based on the above, it is possible to control what portion of cobaltlayer 1100 will be integrated into the resulting liner-free orbarrier-free conductive structure by controlling recess heights R1 andR2 shown respectively in FIGS. 15A and 16A.

In some embodiments, a partial deposition process may be used forruthenium metal instead of the etch-back process described above withrespect to FIGS. 15A-C and 16A-C. For example, instead of depositingruthenium metal 110 to completely fill opening 600 as shown in FIG. 9,ruthenium metal 110 can be deposited to partially fill opening 600 asshown, for example, in FIGS. 17A and 18A. In some embodiments,deposition height H_(d) of ruthenium metal 110 in opening 600, as shownin FIGS. 17A and 18A, can be adjusted so that it is above height H ofthe anchor point (e.g., greater than about 13 nm) and below the topsurface of ILD 145. For example, opening 600 can be filled to a heightH_(d1) shown in FIG. 17A, which has an intermediate value between thedeposition heights H_(d) shown in FIGS. 17A and FIG. 18A. As discussedabove, partial deposition for ruthenium metal 110 may be preferred overa polishing process if the volume ratio 600/105 is greater than about0.8. In the case of FIG. 17A, where ruthenium metal 110 is depositedthinner compared to FIG. 18A, cobalt integration into the resultingliner-free or barrier-free conductive structure is possible as shownfrom FIGS. 17B and 17C. Consequently, the amount of ruthenium depositedin opening 600 controls what portion of the deposited cobalt layer 1100will remain in the final liner-free or barrier-free conductive structureshown in FIGS. 17C and 18C. For example, thinner ruthenium metaldeposition results in additional cobalt incorporation into the finalliner-free or barrier-free conductive structure as opposed to a thickerruthenium metal deposition. Since a thinner ruthenium metal depositionresults in additional cobalt incorporation, the liner-free orbarrier-free conductive structure of FIG. 17C would be more resistivethan the conductive structure of FIG. 18C. This is because ruthenium isless resistive than cobalt. Consequently, the resistance of theresulting structure increases as the amount of cobalt increases.

In some embodiments, ruthenium metal 110 is deposited thicker in opening600 having larger critical dimensions and/or smaller aspect ratioscompared to openings with smaller critical dimensions and/or largeraspect ratios. Therefore, it is possible that openings with differentcritical dimensions and/or aspect ratios can end up with a differentamounts of cobalt as discussed above.

In some embodiments contrary to the cobalt layer 1100 shown in FIGS. 11,15B, and 16B—cobalt layer 1100 shown in FIGS. 17B and 18B is partiallysurrounded by ruthenium metal 110. Consequently, the surface areabetween cobalt layer 1100 and ruthenium metal 110 in FIGS. 17B and 18Bis larger than that of FIGS. 11, 15B, and 16B. Cobalt out-diffusion fromcobalt layer 1100 into ruthenium metal 110 during the heat treatment isenhanced for the structures of FIGS. 17B and 18B, which can result infurther reduction of cobalt out-diffusion from cobalt conductivestructure 105.

The embodiments provided with respect to FIGS. 15A-C, 16A-C, 17A-C, and18A-C are not limiting. In addition, the embodiments provided in FIGS.15A-C, 16A-C, 17A-C, and 18A-C can be combined or modified based on thedescription provided above to achieve a balance between cobaltout-diffusion mitigation from cobalt conductive structures 105 andoverall contact resistance.

Various embodiments in accordance with this disclosure describe a methodfor the suppression of cobalt out-diffusion from underlying cobaltconductive structures to overlying ruthenium-filled liner-free orbarrier-free conductive structures. In some embodiments, a cobalt layerformed on the ruthenium liner-free or barrier-free conductive structuresacts as a “reservoir” of cobalt atoms that mitigates out-diffusion ofcobalt atoms from underlying cobalt conductive structures. In someembodiments, the cobalt layer is a sacrificial layer subsequentlyremoved prior to the formation of additional wiring levels. In someembodiments, the cobalt layer is integrated into the liner-free orbarrier-free conductive structure. In some embodiments, the rutheniumliner-free or barrier-free conductive structures are recessed prior tothe deposition of the cobalt layer. In some embodiments, the rutheniummetal is partially deposited prior to the deposition of the cobaltlayer.

In some embodiments, a method includes forming a liner-free conductivestructure on a cobalt conductive structure disposed on a substrate,depositing a cobalt layer on the liner-free conductive structure andexposing the liner-free conductive structure to a heat treatment. Themethod further includes removing the cobalt layer from the liner-freeconductive structure.

In some embodiments, a structure includes a contact with a first metaldisposed on a substrate, a dielectric layer disposed on the contact, anda liner-free conductive structure embedded in the dielectric layer andformed within the contact. The liner-free conductive structure includesa first portion comprising a second metal different from the first metaland a second portion comprising the first metal. The structure furtherincludes a metal oxide disposed on the dielectric layer.

In some embodiments, a method includes forming a conductive structurewith a first metal on a substrate. The method further includes forming,on the conductive structure, a liner-free conductive structure with asecond metal different from the first metal; where forming theliner-free conductive structure includes: depositing an etch stop layeron the conductive structure, depositing a dielectric layer on the etchstop layer, forming an opening in the dielectric layer and the etch stoplayer to expose the first metal, and depositing the second metal topartially fill the opening so that the second metal is in physicalcontact with the first metal, the etch stop layer, and the dielectriclayer. Further, forming the liner-free conductive structure includesdepositing the first metal to fill the opening and annealing theliner-free conductive structure.

It is to be appreciated that the Detailed Description section, and notthe Abstract of the Disclosure, is intended to be used to interpret theclaims. The Abstract of the Disclosure section may set forth one or morebut not all exemplary embodiments contemplated and thus, are notintended to be limiting to the subjoined claims.

The foregoing disclosure outlines features of several embodiments sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art will appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art will also realize that suchequivalent constructions do not depart from the spirit and scope of thepresent disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the subjoined claims. What is claimed is:

1. A structure, comprising: a fin structure on a substrate; an epitaxialsource/drain region on the fin structure, the epitaxial source/drainregion comprising silicon; a contact on the epitaxial source/drainregion, the contact forming a silicide at a top surface of the epitaxialsource/drain region; an etch stop layer on the contact; an inter-layerdielectric (ILD) on the etch stop layer; and a liner-free via,comprising: a tapered portion that extends through the ILD and the etchstop layer; and anchor embedded in the contact.
 2. The structure ofclaim 1, wherein the contact comprises cobalt and the liner-free viacomprises ruthenium metal that incorporates cobalt atoms into the atomicstructure of the via.
 3. The structure of claim 2 wherein the liner-freevia has a volume ratio of ruthenium to cobalt between about 0.2 andabout 0.8.
 4. The structure of claim 1, wherein the anchor is wider thana lower surface of the tapered portion to prevent the liner-free viafrom pulling out of the contact.
 5. The structure of clam 1, wherein thefin structure comprises a plurality of fins and the epitaxialsource/drain region is a merged epitaxial source/drain region thatcouples the contact to the plurality of fins.
 6. The structure of claim1, wherein the tapered portion has a sidewall angle between about 85degrees and about 90 degrees.
 7. The structure of claim 1, wherein theanchor undercuts the etch stop layer by about 4 nm to about 12 nm oneach side of the liner-free via.
 8. The structure of claim 1, wherein atop surface of the liner-free via is substantially co-planar with a topsurface of the ILD.
 9. The structure of claim 1, further comprising ametallization layer at the top surface of the liner-free via, themetallization layer including a metal oxide etch stop layer in contactwith the liner-free via, wherein the metal oxide etch stop layersuppresses diffusion of cobalt from the top surface of liner-free viainto the metallization layer.
 10. The structure of claim 9, wherein themetal oxide etch stop layer has a thickness of about 3 nm.
 11. Thestructure of claim 9, wherein the metal oxide etch stop layer comprisesaluminum oxide (Al₂O₃).
 12. A structure, comprising: a cobalt conductivestructure on a substrate; an insulating layer on the cobalt conductivestructure; and a liner-free ruthenium metal conductive structure havinga tapered portion that extends through the insulating layer and ananchor in the cobalt conductive structure.
 13. The structure of claim12, further comprising one or more additional insulating layers on thecobalt conductive structure, wherein the tapered portion of theliner-free ruthenium metal conductive structure extends through the oneor more additional insulating layers.
 14. The structure of claim whereinthe anchor has a semi-spherical shape.
 15. The structure of claim 12,wherein the liner-free ruthenium metal conductive structure has apolycrystalline microstructure comprising cobalt atoms.
 16. Thestructure of claim 15, wherein the polycrystalline microstructure of theruthenium metal conductive structure comprises grain boundaries and fillvoids.
 17. The structure of claim 16, wherein the cobalt atomsaccumulate along the grain boundaries and in the fill voids.
 18. Astructure, comprising: a cobalt conductive structure coupled to aterminal of a transistor; an etch stop layer on the cobalt conductivestructure; an inter-layer dielectric (ILD) on the etch stop layer; and aliner-free ruthenium via, comprising: a vertical portion that extendsthrough the ILD and the etch stop layer; and a semi-spherical portionembedded in the cobalt conductive structure.
 19. The structure of claim18, wherein the liner-free ruthenium via comprises a largerconcentration of cobalt atoms at a top surface of the vertical portionthan at a bottom surface of the vertical portion adjacent to the etchstop layer.
 20. The structure of claim 18, wherein the vertical portionof the liner-free ruthenium via comprises a different concentration ofcobalt atoms than that of the semi-spherical portion.